This invention relates to the production of polyamides, particularly the production of nylon 6 shaped articles such as filaments produced directly from freshly prepared (nascent) polymer. More particularly, the invention relates to an improvement in the first or hydrolysis step of polymerization of nylon, namely, using a two-stage hydrolysis with different conditions in each stage.
Previous nascent, or polymerization directly coupled to spinning, processes, such as that described in U.S. Pat. No. 3,813,366, hereby incorporated by reference, and in U.S. Pat. No. 3,578,640 also hereby incorporated by reference, produce shaped articles such as filaments (fiber) having higher water extractables content than the similar articles (fiber) produced from polymer chips which were water washed and dried before being melted and shaped as by spinning. A particularly troublesome water extractable component to processors of shaped articles (fiber) are cyclic oligomers of caprolactam. Present directly coupled (nascent) processes produce fiber or other shaped articles having a cyclic dimer content of about 0.5 to 0.7 percent compared to fiber or shaped articles produced from washed, dried, and melted chips of about 0.1 percent cyclic dimer content.
During the initial ring opening of the caprolactam in the hydrolysis process, the opened ring is free to combine with other open rings thereby forming linear and cyclic oligomers. The cyclic dimer of caprolactam thus formed is very stable and has a low vapor pressure so is not removable from the coupled process. The presence of the cyclic dimer in the finished yarn product causes processing difficulties in the customer mill. The mill problems are associated with the cyclic dimer migrating to the surface of the yarn and depositing on the various yarn path components resulting in a situation where the mill process must be shut down to clean the surface as a result of poor yarn quality caused by improper surface frictional qualities which was, in turn, caused by the cyclic dimer buildup. This condition has been called "yarn shedding" and causes the customer downtime on machinery and a reduction of quality of product. Several methods are known to reduce the effect of the cyclic dimer, such as washing the yarn with water and then drying it, or applying a special overfinish to the yarn to enhance the frictional qualities and allow it to be processed. Both of these methods are expensive and, therefore, unattractive to the customer. Therefore, the only practical solution is to reduce the amount of cyclic dimer formed in the coupled polymerization process to a level low enough that its presence is not a factor in yarn quality. See also U.S. Pat. No. 3,287,322, hereby incorporated by reference, for a discussion of oligomers and their effects in yarn.
By water extractables is meant herein the components of the polycaprolactam shaped polymer article (fiber) which can be extracted with water by washing at a temperature of 100.degree. C. for a period of thirty minutes. The components of water extraction are generally caprolactam, linear oligomers and cyclic oligomers of caprolactam.
By oligomer is meant herein a caprolactam compound having 2 to 4 repeating molecular units.
By polymer is meant herein a polycaprolactam having five or more repeating molecular units.